In a typical magnetic electronic article surveillance (EAS) system for detecting magnetic markers, a magnetized marker is placed in a interrogation zone in which an oscillating magnetic field is generated at a frequency "f" (kHz). The EAS system includes a generating coil for generating the magnetic field and a receiving coil for detecting signals generated by the markers. As the field passes a critical value of about 0.1 oersteds (Oe), the magnetic dipole moment of the marker switches and emits a signal. This causes a pulse of voltage to be produced in the receiving coil. Half a cycle later the dipole moment switches back causing a second pulse of the opposite polarity to be produced in the receiving coil. Because the marker is designed to give sharp pulses, the generated signal contains high harmonics, i.e., signals at all multiples of the frequency of the field f. An alarm is set off using a threshold of the higher harmonics, e.g., 9f kHz, 10f kHz . . . 25f kHz. The shortcomings of prior systems are that the signal at these high hrmonics is very small and the amplifier also generates signals of the harmonics due to amplifier non-linearity. A relatively expensive and precise amplifier is needed to isolate the signal from coherent amplifier noise.